1. Field of the Invention
The present invention relates to an apparatus for treating a filter cake created with a suction dryer so that the filter cake can be detached from the suction dryer in a form advantageous with respect to further treatment.
2. Description of Related Art
From the FI patent 61,739 there is known a drying method where the object to be dried is placed by intermediation of a fine porous liquid-saturated suction surface to hydraulic communication with a liquid that is maintained at an under-pressure in relation to the object to be dried. The advantage of this method is that only liquid penetrates the fine porous suction surface.
The FI patent 77,161 describes a suction dryer utilizing the drying method of the FI patent 61,739, which suction dryer is provided with one or several filter elements. The filter elements are arranged in communication with the slurry that contains drying solids. By means of the suction flow passing through the filter elements, on the surface of the filter elements there is formed a filter cake, which is removed therefrom by means of a special mechanical scraper.
When applying a mechanical scraper for detaching the filter cake, part of the filter cake remains in between the scraper and the filter surface. Therefore the FI patent 87,539 introduces a method where prior to starting the removal of the filter cake, a momentous reversed pressure is caused in the suction dryer, so that a thin liquid film is created in between the filter surface and the filter cake. In connection with the creation of the liquid film, gas blowing is directed to the other edge of the filter cake, so that the filter cake begins to detach from the filter surface. The gas blowing directed to the edge of the filter cake can, when necessary, be boosted by means of another gas blowing arranged at the middle part of the filter surface. However, the use of reversed pressure and two-step gas blowing for instance with thin filter cakes is cumbersome, because in between the blowing steps and at the end of the removal of the filter cake, there always is a big risk that the filter cake is broken into small parts. In that case the effect of the blowing is lost, and the rest of the cake may, owing to the reversed-pressure liquid, be moistened so that the total dampness of the filtering product obtained from the suction dryer surpasses the allowed values. Thus the filtering product must be retreated, which increases the filtering costs considerably.
The object of the present invention is to eliminate some of the drawbacks of the prior art and to achieve an improved apparatus for treating a filter cake essentially simultaneously with its removal from off the filter surface of the filter medium.